The invention relates to an optical storage apparatus using a rewritable medium such as CD or MO cartridge and, more particularly, to an optical storage apparatus which can further improve an accessing performance to a high density recording medium.
Attention is paid to an optical disk as a memory medium as a main stream of multimedia which has rapidly been developed in recent years. For example, when considering an MO cartridge of 3.5 inches, in addition to the conventional MO cartridges of 128 MB and 230 MB, in recent years, a high density recording medium of 540 MB or 640 MB is being provided. Therefore, as an optical disk drive, it is demanded that it can use all of the media of 180 MB, 230 MB, 540 MB, and 640 MB which are at present available. In a personal computer which has rapidly been spread recently, a reproducing function of a compact disc (CD) known as a read only medium is indispensable. From viewpoints of space and costs, it is difficult to install not only an optical disk drive for a CD but also an optical disk drive for an MO cartridge as a rewritable optical disk apparatus. In recent years, therefore, an optical disk drive which can use both of an MO cartridge and a CD has also been developed. According to the optical disk drive of the CD/MO sharing type, with respect to an optical system, a mechanical structure, and a controller circuit unit, they are commonly constructed as much as possible so that they can be used for both the CD and the MO cartridge.
In an optical disk drive of 540 MB or 640 MB which enables a high density recording medium to be used, in association with an improvement of a recording density, a track pitch of the medium is narrowed and it is necessary to improve a seeking precision in order to move a beam of an optical head to a target track and to position the beam. To improve a seeking precision, by suppressing a seeking speed, the beam can be stably pulled in the target track. Ordinarily, in the seek control to a target track, for example, so long as a short seek of 50 tracks or less, the seek control by a lens actuator mounted on a carriage which is driven by a VCM is performed. As for a long seek exceeding 50 tracks, the seek control is performed by both of a carriage drive by the VCM and a carriage drive by the lens actuator. In such a seek control, first, a target velocity according to the number of remaining tracks to the target track is generated and a speed control is executed. When the number of remaining tracks up to the target track reaches a value just before one or two tracks by the speed control, a predetermined decelerating current is supplied, thereby performing a decelerating control. When the deceleration is finished, a control mode is switched to a position servo control, thereby pulling the apparatus into an on-track state. In such a seek control, in order to raise a seeking performance in the high density recording medium of 540 MB or 640 MB, it is necessary to decelerate a moving velocity of the beam to a value near the zero velocity by a predetermined decelerating current at a position just before the target track and to control so as to stably pull in the on-track state.
In such a conventional seek control of the optical disk drive, however, when the target velocity of the speed control is set to a slightly high velocity in order to reduce the seek time, the deceleration of the latter half by the speed control is rapidly executed, so that there is a possibility such that a pull-in speed just before the target track largely fluctuates. Therefore, in the decelerating control by a predetermined fixed decelerating current, the deceleration is insufficient and the beam overruns the target track or the deceleration is excessively performed and the beam is reversely returned, so that there is a problem such that it takes a time until the beam is settled to the target track. Although a pull-in speed just before the target track can be stabilized by suppressing the target velocity of the speed control, since the target velocity is low, it takes a time for the speed control. Even if the settlement time can be reduced, the whole seek time becomes long.
Such a problem also occurs with respect to a one-track seek control in which the adjacent track is set to a target track and the beam is moved. In the conventional 1-track seek control, a one-track seek period is equivalently divided into three periods, for example, an accelerating period, a current-zero period, and a decelerating period every 1/3, and a feed-forward control such that predetermined fixed accelerating current and decelerating current are sequentially supplied to the lens actuator is executed. However, accelerating characteristics and decelerating characteristics of the beam by the lens actuator are variable every optical disk drive. When the accelerating current or decelerating current lacks, therefore, the seek time becomes long and, on the contrary, when the accelerating current or decelerating current is too large, the settlement time becomes long, so that there is a problem such that an enough 1-track seek performance cannot be expected.
In the optical disk drive using a changeable medium such as magnetooptic disk, CD, or the like, a track eccentricity amount of the loaded medium differs every medium. The eccentricity amount of the medium is measured at a stage of an initializing process after the medium was loaded and an eccentricity offset current is supplied to a VCM synchronously with the medium rotation so as to set off the measured eccentricity amount. When the track is regarded as a straight line, the medium eccentricity draws a sine curve. Therefore, what is called an eccentricity memory such as a RAM or the like in which sine values using a rotational angle of a predetermined resolution as an address have previously been stored is prepared. A corresponding sine value is read out from the eccentricity memory synchronously with the actual medium rotating position and an eccentricity amount is obtained on the basis of an amplitude measured as eccentricity information and a phase for a rotation reference position. An offset current is supplied so as to set off the eccentricity amount. In the conventional measurement of the eccentricity amount which is executed in the initializing process after the medium was loaded, for example, a lens position sensor to detect a position of an objective lens mounted on a carriage is used and an eccentricity amplitude and a phase are measured from a lens position signal which is obtained by one rotation of the medium in an on-track control state by the lens actuator. Since the lens position sensor, however, is inherently used for a position servo of a lens locking operation to keep the objective lens mounted on the carriage to a zero position (neutral position), a linearity and a resolution of a detection signal for the position are not so high. Since the signal is an analog signal, an error is mixed even when an A/D conversion is performed. There is a problem such that the eccentricity information cannot be sufficiently measured at a high reliability.
In the conventional optical disk drive, a return light from the medium is detected by a 2-split detector and a tracking error signal is obtained from a difference between two photosensitive signals. In this case, in an ID portion of the medium, a zone number, a track number, and the like are recorded by embossed portions called pits, the return light is attenuated by the pits of the ID portion, a fluctuation which drops like noises appears in the tracking error signal, such a fluctuation erroneously becomes a zero-cross point in a low amplitude portion, and the number of tracks is erroneously counted. To suppress the fluctuation by the return light in the ID portion, therefore, by detecting an envelope, a profile of the tracking error signal is smoothed. However, although no problem occurs in the MO cartridge medium of 540 MB or 640 MB for high density recording, in an MO cartridge medium of 128 MB which has conventionally been used, a mirror portion of a mirror surface structure is formed between the ID portion and an MO recording portion of the medium surface. In an MO cartridge medium of 230 MB, a mirror portion is similarly left in an area other than a user region. Therefore, in case of enabling an MO cartridge of a capacity in a range from 128 MB to 640 MB to be used by one optical disk drive, when an MO cartridge of 128 MB or 230 MB is loaded, if an envelope is detected at the time of formation of the tracking error signal, by obtaining a difference between the photosensitive signals of the mirror portions having the same level, the tracking error signal drops in the mirror portion. Further, a signal dropout of an amount corresponding to a discharge time constant due to the envelope detection occurs, the tracking error signal is largely deformed, and the track counting operation by the zero-cross point is certainly erroneously performed. Moreover, in any medium, when the envelope is detected, at the time of a high speed seek in which a zero-cross time interval of the tracking error signal is short, upper and lower peak levels of the tracking error signal itself are envelope detected by the envelope detection, so that there is a problem such that the tracking error signal is lost.
Further, the conventional optical disk drive has a focusing servo to in-focus control the objective lens mounted on the carriage so as to be focused onto the medium surface. In the focusing servo, a focusing error signal is formed on the basis of the photosensitive output of the return light from the medium. However, since the ID portion on the track of the MO cartridge medium has the physical pits, an in-focus position of the objective lens differs from those of the recording surface of the MO portions on both sides. Thus, the focusing error signal changes step by step in front and rear boundary portions of the ID portion for the MO portion and an unnecessary focusing control is performed. For example, in case of the MO cartridge medium of 540 MB, an outer track has 84 sectors and an inner track has 54 sectors and there are ID portions of the number corresponding to the number of sectors. Therefore, the focusing servo frequently operates in the on-track state and there is a problem such that a current consumption due to the focusing servo increases. Although it is sufficient to turn off the focusing servo with respect to the ID portion, if the focusing servo is turned on and off at a high speed in an interlocking manner with the ID portion, it results in that a large disturbance is exerted on the servo system and an automatic focusing function is lost.